Optical and electrical properties of quantum well structures are of great importance for novel semiconductor device applications. An ultimate goal of monolithic integration of optical, optoelectronic and electronic components requires the capability for controllable lateral and vertical modifications of optical constants and electrical characteristics in such components.
The selective intermixing of group III atoms across heterostructure interfaces, especially in GaAs/AlGaAs quantum well structures, has been extensively studied due to the controllable changes in optical bandgap which accompany well-barrier interdiffusion.
Impurity induced compositional disordering (IICD) is a well known technique for enhancing interdiffusion. It has been successfully applied to the fabrication of superlattice and quantum well heterostructure devices. In particular, IICD has been demonstrated in a variety of structures by using a diffusion process, and also by using ion implantation followed by thermal annealing. Experimental results have shown that ion implantation is a suitable technique for introducing many kinds of impurities into quantum well structures to enhance interdiffusion.
In conventional ion implantation intermixing techniques, the range profile of relevance to intermixing is always considered to be the range for energy losses to nuclear damage processes, and the enhancement of interdiffusion is always associated with the presence of implantation induced defects and/or impurities. The mass and energy of the implanted ions are always chosen such that the impurities/damage distribution are spatially peaked in the region of interest for intermixing., such as the middle of a multiple quantum well or a superlattice structure.
Holonyak in U.S. Pat. No. 4,511,408 discloses a method for disordering the layers in a III-V heterostructure by implanting ions directly into the sample (region of intermixing) and then subjecting the sample to the thermal annealing. As noted in column 4, lines 30-34 the implantation causes considerable crystal damage in the structure, that in this case was a superlattice. The above method for enhancing interdiffusion at heterointerfaces is also disclosed by Hirayama et at. in "Ion-Species Dependence of Interdiffusion in Ion-Implanted GaAs-AlAs Superlattices", Japanese Journal of Applied Physics, 24, pp. 1498-15023 (1985), and by Cibert et at. in "Kinetics of Implantation Enhanced Interdiffusion of Ga and Al at GaAs-Ga.sub.x Al.sub.1-x As Interfaces", Applied Physics Letters, 49(4), pp. 223-225(1986).
Hirayama et at. implanted a group of samples containing superlattices with a variety of ion species and then measured the photoluminescence peak shifts while the samples were being annealed. As in Holonyak, the ions were implanted directly into the superlattice structure, causing crystal damage in the region of intermixing that required thermal treatment to effect recovery. Hirayama et at. observed that the spectral width of the superlattice structure increased after annealing, and explained that the increase is partly due to the inhomogeneity of the interdiffusion resulting from the inhomogenous depth profile of implanted impurity density. Cibert et at. presented spectral measurements of GaAs quantum well structure implanted with Gallium ions and subsequently annealed. Disadvantageously, the implantation caused damage centered on the quantum well and extending deep into the barriers. In fact, the maximum damage from one of the higher ion doses was almost enough to produce amorphization.
In an attempt to obviate the disadvantageous of the above mentioned prior art, Elman et at. in U.S. Pat. No. 5,238,868 describe a method of selectively tuning the bandedge in a quantum well heterostructure. The method steps include implanting ions into the heterostructure to form a disordered region near the upper surface of the sample. Vacancies and defects created by the implantation step are spatially separated from the quantum well active region. The heterostructure is then thermally annealed so that the vacancies diffuse through the active region (quantum wells) and enhance interdiffusion at the heterojunctions thereby modifying the optical bandgap of the quantum well layers. This bandgap tuning is somewhat selective because the implantation step can be to an extent controlled. Although the method described by Elman et at. provides fair results, large energy shifts necessary for many envisaged applications cannot be attained using this method. For example, experiments have shown that by using a method (focused ion beam), similar to Elman's, as the ion implant dose increases, the bandgap change of a quantum well increases monotonically until it saturates, after which, a decrease in the bandgap energy occurs. This saturation takes place because above this dose, there is excessive damage at the surface of the structure that ion channeling becomes difficult and it is no longer possible to deposit ions directly into the quantum wells. A similar effect occurs when shallow ion implantation is used to generate single vacancies.
It is therefore an object of this invention to provide a method of controllably tuning the optical bandgap of a semiconductor heterostructure that overcomes these disadvantages and limitations.